GA20 homebrew isn't quite right

[Phil_S]

I finished the GA20 this morning and the good news is that it works. The not so good news is that it doesn't put out anywhere near the volume I expected from a pair of 6V6. Granted, I didn't expect 5E3 levels, but I figured I'd get at least 10W out of it, which is plenty loud in a 12x12 room.

Pictures and reference to the schematic are here:
http://home.comcast.net/~psymonds/GA20.htm

There is at least one known problem, which is, no surprise, that the 6.3V filament winding is only running at 5.15V. It was supposed to support 2.5A. The tube compliment is 2x 6V6, 6SL7, and 6SJ7. For this I get a demand for 1.5A. I guess that is what happens when you roll the dice on a fleebay tranny. I will manage this somehow and I don't believe it is the real problem with the amp.

In stark contrast, there was no representation made about the 5V winding and it is providing 5.45V to the 5Y3GT. I assume this is not a real problem.

Here are some voltage readings.

No tubes in the amp: 6.3V winding at 5.76VAC.
HT winding 616VAC.
With only 5Y3 in socket, HT winding is 403VDC at the first filter cap.

With the 6V6's added, VDC:
Plate 293
Cathode 12.05
Screen 247
Filaments 5.4VAC (remember preamp and PI are missing)

2nd filter cap 246
3rd filter cap 148

Adding the 6SL7 (PI) and 6SJ7 (V1)...

6SL7 (paraphase inverter)
pin 6 cathode 1.44
pin 5 plate 41.7
pin 4 grid 1.4, this one takes the input from the volume pot

pin 3 cathode (wired parallel w pin 6)
pin 2 plate 151
Pin 1 grid in the mV range

6SJ7
Plate 147
Cathode ~0 (tied to G3)
Screen 19.8
Grid ~0

The best I can figure it that the 6SJ7 from the bone pile is a dud. Does this make sense? I'll have to scrounge around for another?

I took the 6V6s from my 5E3. I know they are good.

The 6SL7 worked find in another amp.

Since the 5Y3 is performing, I think it's good.

Any thoughts?

Phil

[Bob-I]

You're dropping over 100V with 2 6V6's, that's not good. I'd say you either have a bad rectifier, PT or something is wired incorrectly.

Looking at your pics, some of the solder joints look... not to be negative.. sloppy. It almost looks like some of them have the solder making the connection instead of the wires. Make sure they're all clean.

[Andy Le Blanc]

I had a similar issue with a project, the PT wasn't rated for the load.
Not enough ma. to make the watts, scab in a known tranny on the bench,
see if you can get it to go with a known ma. rating.
Its cheaper to get fender replacements in the long run. If the trannies are
from an amp with a known tube compliment you still make assumptions
about the pt secondaries, sometimes the secondary winding insulating layers
are nearly cooked and don't show until after installation etc...

That's the thing with trannies you can't cross reference, it's very easy to screw
yourself into a corner with a project. On top of any other issue that might
pop up because of bad parts, solder joins, dress tubes, etc...

[Structo]

Phil, looks like a cool build but I was curious about why you chose the PTP style vs a terminal board with eyelets or turrets?
Was this to lower the cost or did you just want to do it that way?

To me, PTP looks like it would be hard to troubleshoot if you had noise issues or hum.

I hope you get it sorted, I have that 6V6 amp I converted from a Hammond organ donor and it is loud as hell!

[jjman]

Having less than 6.3vac on the heater winding with no tubes in is a red flag. I suspect a bad power tranny or filter cap(s.) I would disconnect the secondaries on the PT and re-check the 6.3 winding. It should be about 6.5 or more with no load.

[M Fowler]

Phil good to see you at the PTP amp I really enjoy builing PTP amps. I have attached a better schematic as well.

I am still looking things over had to watch my 7 month old grand daughter all day she wore me out.

I'm wondering about the ground paths and thats where I am concentrating my attention to your build right now.

Mark

[Phil_S]

Greetings all. Lot's of good comments and much appreciated.

This particular amp was nearly 100% bone pile built. The goal was make what you can with what you've got!

Bob-I: Thank you for the feedback on the solders. I am very aware of what they look like. I basically finished this on Friday and returned to it on Saturday for final touches. Allowing the night between forced me to take a fresh look at the work. There are clearly a few solders that need touch up, but many of them look a whole lot better on the blind side. I know they are not mil spec and I should have crimped and clipped wires better. I tried but found that I needed three hands for some of them! The main factor is tight working space and wire that won't cooperate. I probably needed some encouragement here, and will go around a second time to touch up and the like. Good eye, Bob.

Double thanks on the rectifier comment. I wondered about this and just assumed the PT was rather "soft". I don't seem to have another 5Y3 around but I've got a 5U4 and I might put that in. I had expected B+ of around 320 or so. I appreciate the clarity you offered on this point. It is easy to be blind when you are the builder.

Andy: I agree that this PT turned out not to be the best choice. I'm going to look into something else. I took the seller (eBay) at his word, and didn't do my usual testing. He said it is 290-0-290 @ 150mA and 6.3 @ 2.5A. Either I've got more than one miswire (I don't believe I've miswired) or this isn't as represented. I don't spend much for iron on eBay, so I haven't lost much, just time. When it comes to hobby time, any time at the bench is good time.

Tom (Structo): See above on style of the build...bone pile = lowest cost. This was a personal challenge. As you know, it is easy to solve problems with money and I am fotunate to be able to make that choice if I want to.

PTP is particularly hard to trouble shoot when you are not the builder and maybe not much better when you are. So far, there is no hum, so let's keep fingers crossed!

jjman: Agreed about the heaters, see above. See the rectifier comment above (Bob-I), and PT comment (Andy).

Mark F: Thanks for the PDFs. It was a bone pile build in a small chassis. Nice layout and a nice clear schematic are still nice to have. Part of this project was just to have the experience of doing a PTP build.

Regarding the grounds...there is a buss that begins at the input jack and follows from there to the first and second pots. There are a few parts that are attached to the buss or to the pots. There is a black wire (4th pic) from the pot to the chassis ground. The 2nd and 3rd filter caps and hung on terminal strips and there is a black wire from the neg side to the chassis ground (2nd to last pic and 4th pic). Everything else goes directly to the chassis ground...just follow the filter cap from the rectifier socket to find the chassis ground. I know I may have broken a few rules here, but giving the simplicity of this amp, I figured I might get away with it.

Thanks, again, everyone, for your thoughtful and constructive comments. It might be a while, but I'll post the follow up.

Phil

[Rick]

It seems strange the fil winding would be so low, a lot of older PTs are wound for 115 or 117 VAC primary at most, so it's usually the case that when connected to 120VAC mains, they run a tad too high, not a volt or so too low. I would be very suspect of the PT, 2 cents FWIW. A known substitute PT should get to the bottom of it quickly. Good luck,
Rick

[Bob-I]

Bob-I: Thank you for the feedback on the solders. I am very aware of what they look like. I basically finished this on Friday and returned to it on Saturday for final touches. Allowing the night between forced me to take a fresh look at the work. There are clearly a few solders that need touch up, but many of them look a whole lot better on the blind side. I know they are not mil spec and I should have crimped and clipped wires better. I tried but found that I needed three hands for some of them! The main factor is tight working space and wire that won't cooperate. I probably needed some encouragement here, and will go around a second time to touch up and the like. Good eye, Bob.

I don't want to sound negative, but I'd recommend that you take a look at the "Dumble now" thread on the Dumble page. There's a link to NASA soldering guidelines. Honestly, some of your connections need more than a touch up. A bad solder joint can kill an otherwise great amp, I've been there.

For example, on the first picture there's a green wire (I think it's green, I'm colorblind) connected to a pot, and it appears that there's a second wire from a cap connected not to the pot, but to the green wire. That should be taken apart, remove all the solder, connect both leads to the pot correctly so that both leads touch both sides of the pot terminal, and solder. Sometimes a joint like that can take 10-15 minutes to get right, but it'll never give you trouble after that.

All that said, I still suspect something more wrong, most likely related to supplying power, either the PT, filter caps, dropping resistors.

Good luck.

[Phil_S]

Bob: I don't think you are being negative at all. A good critique is always worth giving. It's apparent, I'm not calling a spade a spade. You're just keeping me honest and I appreciate that.

I read the mil spec soldering document to which you refer when the Dumble thread was fresh. It was a real eye opener and it is how I know some of my solders really do look like crap. I think you are right about this and I'll need to go back through the amp. BTW, at that pot, there is a green wire and a cap, no second wire. There is a close up (not yet soldered) about the 7th picture.

The PT that's in there is running about 285-285 with the load of the circuit. Swapping in 5U4 gave plate voltage of about 310 (+~20V). I didn't have a chance to run a guitar through it yet. I should do that before I attempt a tranny swap. It's clear the PT has to be swapped as the filament supply is not what's needed.

I've got a 335-0-335 in my bone pile. I was thinking it will be too much B+, around 400V on the plates. The caps are rated OK, but it is this an unwise choice for the circuit?

Thanks.

Phil

[badtweed]

Just a thought, any chance that you have the 5 volt and 6.3 secondaries mixed? Meaning do you have the rectifier hooked up to the 6.3 secondary by accident and have the 5 volt hooked to the heaters instead?

[Bob-I]

Bob: I don't think you are being negative at all. A good critique is always worth giving. It's apparent, I'm not calling a spade a spade. You're just keeping me honest and I appreciate that.

Great, I'm sure now you'll get those solder joints done perfect.

But you've still got the voltage drop problem.

[Bob-I]

I've got a 335-0-335 in my bone pile. I was thinking it will be too much B+, around 400V on the plates. The caps are rated OK, but it is this an unwise choice for the circuit?

Thanks.

Phil

Maybe I'm missing something, but didn't you say you were getting 400 at the first filter but only ~300 with the 6V6's in? That's a bad sag, plus the low heater voltages all point to a bad PT.

I'd wire up the other PT without mounting it and see what the results are. If your heaters are correct, and the B+with the 6V6's is about 360-380 depending on the rectifier, you're golden.

[KT66]

A couple of things stand out to me that don't add up. One of them is that you state that before you plugged in the preamp tubes that the voltage measurements at the 2nd and 3rd filter caps are 246 and 148 ; However, with no current draw there should be no voltage drop across the 47K power resistor. Also, the PI has a common cathode and identical plate resistors so the plate voltages should be the same, but you recorded 151 and 41.7. I would suggest that there is some kind of short that is causing both the voltage drop across the PS resistor with no tubes in and the diminished plate voltage on pin 5 of the 6SL7.

I also agree with Bob that dropping 100V when the 6V6 tubes are put in seems excessive. Did you measure the plate voltage on both tubes and are they about the same ? If you look at the schematic for the GA-20T, which has the same power section, there is 90V difference between the screen and plate, your amp has half that which might point toward the excessive 100V drop.

A word of caution also, the 5U4 draws 3 amps of current and if your tranny was designed for 5Y3 use (which uses 2) you might be in danger of toasting the 5V winding.

[Structo]

Yes, I was going to mention that about the 5U4.

I never messed around with tube rectifiers before I built the 6V6 amp from a Hammond donor.

I draws a lot of current and also drops the B+ ~50v.